Today's remote tagging (or tracking) systems generally fall into one of two categories. They either comprise a tagging device that is “active” and sends out a signal to a detector, as seen in UK patent application GB2383216 for example, or a tagging device that is “passive” and hence needs to be detected by an active detector signal, as seen for example in International Patent application WO03/096053.
Both types of existing tagging systems have been implemented successfully and, as a result, their use is now relatively widespread. Various objects are tagged so that a remote tracker can either follow their movement or monitor the fact that they are not moving. Tagging systems have, for example, been used in the fields of biology (to track the movements of animals), search and rescue (to find victims in remote areas), and exploration (to enable separated groups to stay in touch). The main area of application for remote tagging systems is however the field of security: the tagging of vehicles, for example, allows car thieves to be apprehended more easily, whilst tagging prisoners enhances the security of prisons or even enables convicts to be monitored at home. Tagging devices (tags) are also used in more sophisticated ways, for instance to help secure boundaries by guaranteeing that the components of boundary security systems cannot be removed unnoticed.
In many of the tagging applications relating to security, and indeed in some applications in other fields, it is essential for the tracker to be able to authenticate the information received from the tag. Users of a tagging system often need to be entirely certain that the information obtained from a tag is correct and has not been tampered with. Similarly, it may be important that the flow of information between a tag and its tracker is not meaningful to an eavesdropper, for instance if the owner of the tagged object wishes to keep his or her identity under wraps. Accordingly, there is a need for secure authentication systems that guarantee the validity and integrity of information received from the tag and ensure that any communications that are intercepted are of no use to eavesdroppers.
A number of existing systems aim to provide secure authentication of a tagging device's position. Most of these systems attempt to mitigate the problem of potential tampering or eavesdropping by securing communications between the tag and the tracker through cryptography. Location information sent out by the tagging device is encrypted using an encrypting algorithm and a secret encryption key, and is eventually decrypted by the tracker with a decrypting algorithm and a decryption key. Unfortunately, although such encryption systems can make it harder for communications between the tag and the tracker to be understood and/or faked by eavesdroppers, there are a number of ways in which their security is flawed.
Firstly, since the encrypting and decrypting algorithms used in classical authentication systems are generally publicly known, secure authentication is rendered impossible as soon as the eavesdropper knows either the encryption key or the decryption key. An eavesdropper equipped with the correct key can decode messages and/or send fake (or spoofed) signals to give the tracker incorrect information concerning the tag, allowing the real position of the tag to be tampered with unnoticed.
Encryption and decryption keys can for instance become known to eavesdroppers if there is momentary access to the tagging device itself (which houses at least the encryption key) or if the entire encryption system is cracked using the information travelling from tag to tracker or tracker to tag. As the processing power of computers increases, it will become easier to crack even relatively sophisticated classical encryption. Any encryption based on classical information thus has a fundamental flaw in that senders and recipients have no way of being entirely sure of whether or not any eavesdropping has taken place. Existing authentication systems can never give users complete peace of mind, since it is in theory possible to crack any classical encryption.
In addition to the problems encountered in the event of a key becoming known it may even be possible to fake the tag's signal without cracking the classical encryption. Depending on the precise working of the classical tracking system, it may be possible to record and play back encrypted information sent to the tracker in the past to give a wrong impression of the tag's current location (a so-called spoof signal).
Furthermore, tracking systems relying on classical encryption possess another disadvantage in that they require the tagging device to have enough processing power to encrypt or decrypt information. This not only increases the size of the tags but also has an effect on the cost of the system. There is inevitably a trade-off between cost/convenience and security, since more advanced encryption algorithms require more processing power and therefore make tags bulkier and more expensive.
The present invention aims to overcome at least some of the problems described above by providing a truly secure method of authenticating the position of a tagging device. The present invention has arisen from the appreciation that whilst authentication systems using classical information can never be considered entirely secure, it is possible to use relativistic signalling constraints and quantum information to achieve extremely high levels of security.
The invention described herein is to a large extent based upon quantum mechanics, quantum information and quantum computation. Some of the fundamentals of these fields can be acquired from “Quantum Computation and Quantum Information” by Michael A. Nielsen and Isaac L. Chuang (henceforth referred to as “Nielsen and Chuang”). In particular, Nielsen and Chuang contains information regarding entanglement and the properties of qubit pairs that are in one of the four Bell states (referred to as Bell pairs in this specification). It also familiarises readers with notations conventionally used in the field of quantum physics and provides ample references to other texts that cover specific areas in greater detail.